Every forest includes a host of microorganisms, insects, and pathogens (disease-causing organisms) that cause damage to the trees within it. A certain level of damage is unavoidable, and can even be healthy for a forest ecosystem. Therefore, modern management techniques must include the consideration of protection against forest pests. In this activity, you are a forester figuring out strategies to deal with a forest pest, quantifying the damage to your plantation, and considering the costs and benefits of different strategies. This activity is adapted with permission from Biology in a Box

VocabularyMontotypic stand: A group of trees in a given area all of the same species. Usually this situation is man-made.Mixed stand: A group of trees in a given area of more than one species.

Introduction
The relationships between forest trees and their main pests have evolved over many years. Epidemics or catastrophes are normally rare. However, poor management techniques and the introduction of organisms from other countries can devastate a forest ecosystem. They can even eliminate a specific tree species from a region. Since humans can change the landscape in such a short period, we sometimes interrupt the natural defenses that evolved within the system we are changing. In the case of exotic pests, the forest has not evolved with these organisms, and may be defenseless against attack.

Photo credit: Roger Anderson, Duke University, Bugwood.org

Various organisms, including viruses, bacteria, fungi, nematodes, parasites, and insects, cause diseases in forests. Damage may occur to the leaves, roots, or stem tissues, which severely reduces the tree’s ability to function in a healthy manner. Foresters continuously work to try to stay ahead of any impending attacks. The southeastern forests have their share of challenges. The plight of the American chestnut gave biologists a look at what can happen if exotic organisms are left unchecked. The chestnut once dominated southeastern forests, but there was an accidental introduction of a fungus called chestnut blight that killed nearly all adult chestnut trees in the early 1900s. Now American chestnut trees exist in forests only as sprouts from stumps of long dead trees, though some people are attempting to breed a blight-resistant tree that can be re-introduced into the wild. Pests that are on their way or are already a problem in southern forests number in the hundreds, including dogwood anthracnose (another fungus) and hemlock woolly adelgid (an insect).

Landowners can have a variety of goals for their forests. They may want to manage for timber to sell, to manage the forest for recreation, hunting, aesthetics, or habitat quality, or they may simply want to maintain it for sentimental reasons. This activity presents a simplified exercise for one struggle that a plantation manager might face. It will help you to understand some of the factors that influence the level of damage within a forest. You will learn some simple methods of management that can help improve forest resistance to disease.

The Southern Pine Beetle (Dendroctonus frontalis, see figure) is a native forest pest of southern pines. It is considered the most destructive pest of pines in the southeastern region. Adult females bore into the trees and send out pheromones that, together with chemicals of the tree, can attract a large number of other beetles. These aggressive beetles can attack and kill large tracts of pine in one growing season. After killing their host tree, they will spread to neighboring trees. Outbreaks generally occur every 7-10 years. Most infestations are limited to small areas, but occasionally these infestations can sometimes kill thousands of acres of pines.

Inside the inner bark of the pine tree, the beetles feed on the phloem tissues, forming serpentine galleries (see figure). Both the adults and larvae feed in this way, which essentially girdles the tree (cutting off the transport of sugars) and eventually kills it. One external sign that a tree is infected is the presence of white globs of resin that look similar to popcorn.

After an infestation of pine beetles, forest managers cut the infected trees and a buffer of surrounding healthy trees in attempt to stop the invasion (see figure). This can be challenging for managers as the adult beetles can fly up to around 2 miles.

One strategy to prevent massive tracts of trees from being infected is to plant zones of a different species of tree within a plantation. Many pests are specific to one host species, so alternating with another species can help stop a disease from potentially spreading widely, isolating it to one area where the infestation starts.

The Scenario: Imagine that you own a tract of land planted with pine. Your plan is to sell the pine to pulp and timber companies. Your challenge as a land manager is to maximize your profits while faced with possibility of beetle infestation. Beetles spread to surrounding trees with each generation. For simplicity’s sake, we will assume that the beetles take one year to do so, but in reality they may spread even faster!

The trees are worth more every year you let them grow larger, but if they become infected with beetles they will be worth less. For example, in the first year, you could get $100 for each patch of pines, but if you let them grow for 4-6 years each patch will be worth $250. However, if they become infected in years 4-6, they will only be worth $70. See the table below.

Year

Price for uninfected pine patch

Price for infected pine patch

1

$100

$40

2

$150

$50

3

$200

$60

4-6

$250

$70

Infestation in a Monotypic Stand

What to do

Find the game board covered only with light green tree crowns that looks like the one below. This represents your newly-purchased land, which has 36 plots of pine on it.

There is an outbreak of Southern Pine Beetle in your plantation!

Now choose any plot of pine on the plantation and place a paper clip on it. This represents a plot in which a beetle infestation begins.

In the next generation of this infestation, each pine tree plot adjacent to your infected plot is now infected. (Any plots touching the infected plot, whether above, below, on either side, or even diagonal,are considered adjacent.) Place a paper clip on each of these plots.

Example of spread of infestation

Make a table like the one below. We have already filled in data for generation 1 of the infestation.

Year

# Healthy

# Infected

Profit if Harvested

1

35

1

35 × $100 + 1 × $40 = $3540

2

3

4

5

Record the number of plots remaining uninfested and those infested for the second generation of beetles. Calculate your potential profits.

Repeat the pattern of spread of beetle infestation for a total of 5 generations of the beetle, filling in the cells of the chart as you proceed.

See how much of your plantation remains healthy after a total of 5 generations of beetle infestation.

So … what happened?

1. Describe what happened to the pine plantation.

2. What is the health of the plantation in year 5?

3. Given this infection, in what year should you harvest to maximize profits?

4. What is a disadvantage to having a monoculture, or a forest with only one tree species?

5. What are some possible solutions for keeping an infestation contained?

6. Where would you want the infestation to start in order to have the most healthy plots after 5 rounds?

In the example graph “Number of Infected Plots”, a possible infection scenario has been plotted over time. Use this example to answer the following questions:

7. In what year does the infection begin to level off?

8. On a separate sheet of paper, copy and fill in the corresponding table, including calculating the profits by year. In what year should the plantation manager have harvested to maximize profits?

9. Looking at the game board, can you figure out where the infection may have started?

Year

# Healthy

# Infected

Profit if Harvested

1

35

1

35 × $100 + 1 × $40 = $3540

2

3

4

5

Infestation in a Mixed Stand

Now find the game board that has alternate rows of dark green tree crowns and light green tree crowns.

The light green tree crowns are your pine plots (the cash crop). The dark green tree crowns represent plots of hickory trees that offer no cash reward (at least not for many more years than pine, because hickory is slow-growing). However, note that hickory is not attacked by the Southern Pine Beetle.

Repeat the same invasion process you followed in Exercise 1: Choose a random pine tree plot as a point of beetle infestation and proceed through 5 generations of beetle damage. Record your infestation in a table like the one below.

Year

# Healthy Pine Plots

# Infected Pine Plots

Profit if

Harvested

1

2

3

4

5

Now answer these:

10. What differences are there after 5 generations of beetle damage as compared to your monotypic stand?

11. What did planting alternate rows of hardwoods trees do for your pine trees?

12. What are other management techniques that might result in the same outcome?

13. What happened to your pine tree profits in terms of selling harvested trees to timber and pulp companies?

14. What are you compromising in trying to manage against Southern Pine Beetle damage?

Answers: 1. Answers will vary, but overall, most or all of the forest should have become infected by the fifth round 2. Not healthy at all! 3. This will vary depending on where you selected for the infestation to begin. Look for the highest value in the “Profit if Harvested” column. It may not be what you would expect.4. A monoculture plantation is devastated if a pest attacks it. Since all of the trees are the same, all of the trees are likely to be affected. 5. Plant buffers of either trees that are resistant to pine beetles, or plant in a scheme using gaps that are larger than the dispersal ability of the female beetles. 6. One of the four corners would be best. If you were incorrect, try this out to see for yourself. 7. The number of infected trees levels off at year 4. 8. The manager should have harvested in Year 2, when profits would have been at their highest, at $4500. (see table)

Year

# Healthy

# Infected

Profit if Harvested

1

35

1

35 × $100 + 1 × $40 = $3540

2

27

9

27 × $150 + 9 × $50 = $4500

3

16

20

16 × $200 + 20 × $60 = $4400

4

0

36

36 × $70 = $2520

5

0

36

36 × $70 = $2520

9. There are a few possibilities that would work, but one is row 2, column 3. Actually, it’s just like in the “Example of spread of infestation” figure. Since the number of infected trees went from 1 to 9 in the second generation, you know the infestation had to start on one of the interior plots, since only those plots would have had enough neighbors to which the beetles could have spread in one generation.10. A greater number of the trees are still undamaged. 11. It provided a buffer of trees that are resistant to the beetle. This kept the beetles from infecting all of the pines. 12. Managers can create gaps that are larger than the beetles’ dispersal capabilities. 13. In the short term, profits are lower because fewer of the trees can be harvested for profit. But in the long term if there is an infestation, profits are better if management techniques are used to guard against beetle attack. In this example, the maximum loss of trees to infestation would be 5, as opposed to all 36. More of the trees are able to grow to a higher value. 14. It’s a tradeoff between planting fewer pine trees and earning less money if there is no infestation or planting a monoculture of pine trees and losing more trees overall if there is an infestation.

Extended Links:
The Blight Of the Hemlock: NIMBioS Songwriter-in-Residence Jay Clark debuts his song, The Day the Last Hemlock Died, which tells the story of the blight of the Eastern Hemlocks in Southern Appalachia. The hemlock is another tree battling its own enemy: the woolly adelgid. http://www.nimbios.org/videos/hemlock

Bark and Wood-Boring Beetle of the World: website devoted to providing information on the many different beetles that like to chew on trees. http://www.barkbeetles.org/

Southern Pine Beetle Prevention Program: The USDA Forest Service website for their program to control the southern pine beetle. http://www.fs.fed.us/r8/spb/

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In Biology by Numbers, learn about the ways math can solve biological problems. Produced by the National Institute for Mathematical and Biological Synthesis (NIMBioS). NIMBioS brings together researchers from around the world to collaborate across disciplinary boundaries to investigate solutions to basic and applied problems in the life sciences. NIMBioS is sponsored by the National Science Foundation, the U.S. Department of Homeland Security, and the U.S. Department of Agriculture with additional support from The University of Tennessee, Knoxville.